Researchers from the University of Illinois in the US?have published their findings on a colour-changing polymer that they claim will allow mechanical damage, such as the stress of a load, to be used for useful purposes.

On a basic level, this means the polymer changes colour as a force is exerted upon it and allows the degree of damage to be seen on the material.

This offers a number of applications, with the potential use as an element of the ropes used in the lifting industry, where inspection and maintenance are integral to the safety of lifting operations, one option.

The team, including researchers from three groups at the university funded through a multidisciplinary university research initiative from the Army Research Office/Army Research Labs, said the idea behind the polymer was based on the desire to investigate new ways of generating a chemical reaction.

This extends their existing work on self-healing materials, such as epoxy coatings, fibre composites and rubbers, and develops the positive use of mechanical damage.

“Most people are familiar with the idea that you can induce a chemical reaction using heat or light,” says Doug Davis, a graduate student from the univeristy and one of the authors of the report on the findings. “But the use of direct mechanical force to initiate a chemical reaction is not as well known.

“Previous examples have included the grinding of compounds together to make them react, subjecting polymer solutions to shear or sonication, or tests on single chains using atomic force microscopy tips to select and stretch individual chains.

“Rather than focus on how currently available materials or compounds reacted to mechanical force, we wanted to design reactive units (mechanophores) that would respond in very specific and useful ways.“

Davis says the process works by covalently [see Definition of terms panel] incorporating spiropyran (the aforementioned mechanophore) into the polymer. The spiropyran is either merged into the polymer chain or acts as a cross link between chains.

The spiropyran used in the process is formed of two perpendicular halves joined by a spiro carbon. There is no conjugation between the two halves, which means they absorb light in the UV spectrum. When the central, covalent spiro carbon-oxygen bond is broken, the molecule twists to a planar, conjugated form, and this conjugation shifts the light absorption into the visible spectrum, causing the colour change.

“Our aim was to break this bond by literally pulling it apart,” says Davis. “The molecule is obviously too small to do this normally, so we make figurative handles by attaching long polymer chains to it. This bulks the molecule up and allows them to be manipulated macroscopically. Polymer chains were attached on either side of the bond we wanted to break, so that when the polymer is stretched, the force is transmitted through this bond.”

The development of the colour-changing polymers still faces a number of issues, says Davis, and is not yet ready for commercial applications. Issues include threshold strain, with an upper limit on the colour change based on the number of spiropyrans available in the polymer; light and heat sensitivity; and bleaching, although this is dependent on the ambient light.

Davis says work is being done to overcome these problems, with the potential for UV filters to mitigate light sensitivity and changes to the spiropyran or polymer matrix to reduce the impact of heat.

Overcoming these issues could pave the way for the technology to be integrated into products such as composites and coatings, and therein some of the ropes and wires used by the lifting industry.

Davis says initial applications would be in damage detection, but along the lines of shock or tilt indicators found on shipped packages, which allow a certain threshold of damage to be detected. Other applications would be in analysing stress and damage in materials manufactured from the polymers.

“I don’t think we’d thought specifically about the lifting industry,” says Davis. “However I can definitely see the applications. While not exactly the same thing, I’m reminded of climbing ropes which need to be inspected for damage often and are rated for a certain number of falls.

“Technology like this might lower costs by preventing the premature discarding of rope/cable that is still in good condition as well as improving safety, since damaged rope/cables could be easily identified.”

Another way

Roland Verreet, managing director of Wire Rope Technology, says the potential for the research in the US to be integrated into products for the lifting industry is beset by one main problem – that running ropes cannot be coated. This means the colour change check could only be used after the rope has been overloaded.

“It’s a very interesting product but the problem with wire ropes is that running ropes can’t be coated,” he says.

“So it’ll either have to be on the inside where it can’t be seen or individual wire ropes replaced by plastic ropes, which poses different problems with different elasticity. It will be difficult to use with steel rope.

“If you had it in the core you could use it to see if the rope has been overloaded. But it couldn’t be used for inspections. That’s my feelings on it, although you can never say never.”

He says if the issues can be overcome then it will “be great” and a welcome addition to the market. “The failure mode of wire rope is complex so it could be used with every other tool that is available.”

To this end, Verreet says the Institute of Mechanical Handling and Logistics at the University of Stuttgart in Germany has developed another inspection and detection system.

This is an optical detection system that uses a computer system to analyse captured images of a rope to compare them against a control image that represents the ideal rope condition when it has just come off the production line.

Verreet says lay lengths of a rope are essentially identical, meaning you can look at one section and it should be the same as any other on the rope. Moreover, it means you can compare them to the control section and see how the rope changes during its lifespan.

Cameras with a high resolution down to 0.1mm are used to record images of the rope on four sides, which are then fed into specially designed computer software that wraps them to create a full representation of the rope.

This is then recorded and saved for review and allows changes in the rope to be seen, such as thickening, thinning and stretching.

“It will only see the surface of the rope but if the inside changes then the outside is affected as well so you can see what’s going on,” says Verreet.

By reviewing the recorded images against the control, the software is able to detect changes in the rope’s shape and any potential problems. Users can then zoom in and out on the image to carry out more in-depth checks.

The imaging system also breaks the rope down into lay lengths allowing the user to call up specific sections of a rope if they so desire. This means it can be used to check specific areas of a rope in the event of a failure or accident.

Verreet adds that the system can be used to record historical data, which means users can review the changes in a rope over a set timeframe and see what happened and when.

“I did a presentation on this technology and used a demonstration of a boy who took a picture of himself every morning for eight years. This showed you how he changed, when he had his hair cut, when he grew a beard, when he met a girl, when he got into punk.

“It is essentially the same thing.”

The system has been in development for a number of years, says Verreet, with the first prototypes now being tested on tramways. Verreet is currently applying for a patent for the system.

“It is a very interesting development.”